As is known in the art, memory cells are used to store (i.e., read and write) bits of digital words. Non-volatile memory cells are able to store the bit without sustaining electrical power. One type of memory cell is a MEMS memory cell. One such MEMS memory cell is described in U.S. Patent Publication No. 2001/0018461 published Sep. 6, 2001. Such publication describes nanomechanical data storage device called a “Millipede”, which is using write/read tips on specific polymer, similar to old punch cards but with ability ability to erase data and rewrite the medium. Such device is relatively complicated in design and manufacturing and the use of a plastic media results in the degradation over time at elevated temperature of rewriting.
Another MEMS memory cell is described in U.S. Pat. No. 7,050,320 issued May 23, 2006, such device having a memory layer and a MEMS layer. The storage medium includes chalcogenide islands as storage elements. The MEMS layer includes a movable MEMS platform having probes to connect selected chalcogenide islands via positioning of the MEMS platform. A high voltage source disposed external to the memory layer and the MEMS layer provides a high voltage to a stator electrode on the memory layer and to a rotor electrode on the MEMS platform to control movement of the MEMS platform with respect to the storage medium.
As is also known in the art, certain alloys known as shape memory alloys (SMA, also known as a smart alloy or memory metal) is an alloy that “remembers” its geometry. After a sample of SMA has been deformed from its original crystallographic configuration, it regains its original geometry by itself during heating (one-way effect) or, at higher ambient temperatures, simply during unloading (pseudo-elasticity or superelasticity). These extraordinary properties are due to a temperature-dependent martensitic phase transformation from a low-symmetry to a highly symmetric crystallographic structure. Those crystal structures are known as martensite (at lower temperatures) and austenite (at higher temperatures). One type of SMA is Nitinol is a Nickel-Titanium alloy with shape memory properties. It was discovered at the Naval Ordnance Laboratory and was released for public in 1961. Nitinol has super-plasticity and shape memory. Below the critical temperature (critical temperature depend on alloy composition), the alloy is flexible and can be bent easily into any shape. This is known as the martensitic state, where atoms are in flexible lattices, which allow the metal to be bent easily. When alloy is heated to the temperature of transformation to austenitic state, it restores the original shape, known as ‘heat memory’, where atoms become locked into their previous rigid arrangement. In our case, when Nitinol memory tip's actuator is heated by electric current into the austenitic phase, the alloy recovers its initial shape.
Shape memory alloys find wide application in the design of solar panels on some satellites where they are raised into position by Shape Memory Alloys (SMA), activated by the heat of the sun. Another, more simple but more exciting examples of Nitinol application is metal-bending trick produced by psychic-magicians. For example, silver plated Nitinol spoon can change its shape by application of heat corresponded by fingers. Additional information about shape memory alloys can be found at http://www.stanford.edu/˜richlin1/sma/sma.html.